If we imagine a situation, lets say a couple of dark colored animals will emigrate to a glacier and then be stranded there.As time goes by, the the population will grow. If we now imagine that they have a selection pressure for being white (for one reason or another), how will this start to happened?

I understand the concept of evolution, but what I don't understand is this:What does it take for an animal to be lighter (Brown->lighter brown->white)?If there is one animal in the population that is a bit lighter than the other animals, I understand that it's offspring has a chance to be a bit lighter too. But how can it be lighter than it's light parent? Does it always has to be a mutation for the evolution to make the animals lighter and lighter? Otherwise I cannot understand how they can be lighter and lighter, unless there is an animal to mate with that is already a bit lighter than the other ones.

How about you think of the situation that there are 2 type of animal, dark and light only... (taking example of industrial melaninsm). In the "white" environment, lighter moth easier to camouflage themself, so easier to survive and selected by natural selection, while darker moth easier been spotted by predator and selected against natural selection.By the way, if you want to discuss in your example, can you tell us what is the animal u are saying?

There will be lighter members in the population, from lesser expression of melanin, but lighter itself has to be useful - it can't be because white will eventually be good. And just as offspring might be taller or shorter than the parents, they can be lighter or darker as well.

Darby wrote:There will be lighter members in the population, from lesser expression of melanin, but lighter itself has to be useful - it can't be because white will eventually be good. And just as offspring might be taller or shorter than the parents, they can be lighter or darker as well.

Is that means under the environmental effect, the animal tend to produce less melanin and become whiter? Is the stages happened during the baby level or as well as in adult?

To magicsiew: "if you want to discuss in your example, can you tell us what is the animal u are saying?"

The "white animal" was just an example.It doesn't have to be the color. The same question is about anything,I understand that the evolution works if there is already variability in the population.

Take for example evolution of a bacteria.The bacteria population can evolve to be resistant to antibiotic, but in that case there has to be some members in the bacteria population that are already resistant to the antibiotic. So the same thing about the lighter animal. How can it be lighter and lighter unless there is already an animal that is lighter than the others to mate with. Or has it to be a mutation every time the "lighterness takes a step forward?"

I see what you trying to say now. Well, in bacteria, is kinda different, as mutation occur quite frequent, especially under environmental stress. It can be the initial population do not has any resistance against certain antibiotic, but the mutation that cause the resistance which later on continue to propagate.

About animal case, I think Darby give a good point. Because you see, if we (human) migrate from a hotter country to a colder country, the skin colour become fairer, same to a person that works constantly in the air-cond room. Under different condition, the expression of melanin differ.

Darby wrote:There will be lighter members in the population, from lesser expression of melanin, but lighter itself has to be useful - it can't be because white will eventually be good. And just as offspring might be taller or shorter than the parents, they can be lighter or darker as well.

it doesn't have to be useful, it just cannot be harmful (or at least not too much;)

Just a few more word:If the character is regulated by a single gene whose correct expression is necessary you are going to have an all or nothing kind of selection, albinism would be a fitting example for you. However this is more an exception than the rule and many characters are governed by more than one gene who all can be mutated. And so can their regulatory sequences. Each different allele will have some extent an effect on the final product. Complex system will likely result in a Gaussian distribution of the phenotype observed. If the character gives an advantage the individuals on one side of the distribution will have a reproductive edge compared to those on the other side, and on the next generation, since the mates were not necessarily all at the exact edge of the distribution what will happen is a gradual shifting of the distribution to wards the favored side of the distribution. The importance and the speed of the shift will depend of numerous parameters including the intensity of the selection, the amount of possible variations, sexual preferences expressed during mating and so on.

In the case of bacteria and antimicrobial resistance it is a bit different, because the population numbers are usually much higher, and the selection is generally of the all or nothing type. And sex or more accurately Horizontal genetic transfer has a very different effect whose importance is quite hard to measure out of the lab, and whose importance is difficult to measure or even guesstimate with any accuracy.

Patrick

Science has proof without any certainty. Creationists have certainty without
any proof. (Ashley Montague)